There is a great deal of interest in polymeric nanocomposites because of their improved properties over other types of polymer composites when combined with a nano-reinforcing material. The most interesting nano-reinforcing materials include layered clays, such as natural or synthetic silicates, and nano-fibers like cellulosic nano-fibers, carbon-nanotubes, metal-oxide nanotubes, to name a few. Among them, polymer/clay nanocomposites have received much attention because of a noticeable increase in physical properties and performance characteristics without sacrificing their processability. Some of these properties include dimensional stability, heat distortion temperature, stiffness, strength, flame resistance, barrier properties, ion conductivity, optical properties, thermal stability, and impact resistance.
However, one of the major drawbacks associated with these nanocomposites has been in their preparation due to the low level of interaction that occurs between the essentially non-polar hydrophobic polymers (e.g., polyolefins (PO) and polystyrene (PS)), or weakly polar polymers (for example, polyamides (PA) and polyesters (PEST)) and the hydrophilic nanoclay surfaces. This low level of interaction between the two components can lead to poor dispersion of the clay material within the polymeric matrix and poor interface interaction between them, resulting in a reduction in the overall performance of the nanocomposites.
A number of techniques have been described for attempting to overcome the poor dispersion of the layered material into the polymeric matrix. The basic procedure includes intercalation of the layered clay material followed by exfoliation within the polymer matrix. Intercalation involves the insertion of molecules, known as intercalants, between platelet particles of the clay material thereby increasing the interlayer spacing (i.e., spacing of the clay platelet in a stack, determined by X-ray diffraction analysis as d001) to at least 1.5 nm. Accordingly, the intercalant has to be able to infiltrate between the layers of the clay material and penetrate the interstices or the clay galleries to render the hydrophilic clay surfaces more organophilic. A gallery is the space between two adjacent platelets, with the gallery spacing equal to d001 less the thickness of the clay platelet. Intercalants that are used for this purpose include polar or hydrophilic solvents, monomers or polymers, inorganic cations, and organic cations, such as quaternary, ternary, secondary or primary ammonium, phosphonium, or sulfonium. Intercalants should bond to the clay surface to ensure that they do not migrate out of the galleries and cause the galleries to collapse during compounding of the nanocomposite. Intercalants are desirable to maximize the interlayer distance between the platelet particles so that they eventually help the exfoliation of the clay into the polymer matrix, which further promotes the benefits brought about by the addition of the layered clay material to the polymer matrix.
Exfoliation of the intercalated clay particles is a process whereby the interlayer distance between individual platelets dispersed within the polymer matrix becomes greater than about 8.8 nm. At this interlayer distance, the desired performance characteristics afforded by the clay material are achieved, while maintaining certain properties that are inherent to the pristine polymer, such as glass-like transparency.
The preparation of a nanocomposite by the melt compounding process as described in WO 00/34393 (Barbee et al.) combines a polymer and a concentrate comprising a layered clay material with a matrix polymer-compatible functionalized oligomer or polymer. The functionalized oligomer or polymer specifically contains an onium group, preferably an ammonium group (e.g., Jeffamines, Etomeens or another modified polymer or oligomer) that provides for increased intercalation of the clay material. However, using molten polymer, as opposed to polymer in solution, leads to poorer and less controllable interaction between the onium group and the clay, adversely affecting the properties of the nanocomposite.
Another technique, which is typically used in conjunction with intercalation and exfoliation, is the use of secondary intercalants or compatibilizers. For instance, U.S. Pat. No. 5,973,053 (Usuki et al.) describes the use of main guest molecules having a polar group in a main chain and/or side chain as extended intercalants. It is essential that the guest molecule possess a polar group (e.g., hydroxyl, halogen, carboxyl, anhydrous carboxylic acid, thiol, epoxy, amino group) to bond to the organoclay surface. Thus, the guest molecule remains in the interlayer section of the clay mineral without being eliminated due to the polarity, thereby allowing the interlayer distance to expand sufficiently. However, Usuki et al. have given little consideration to the effect of the reactivity and structure of the guest molecule on the interaction between the clay and the polymer matrix. In addition, great loss of ductility of nanocomposites using such compatibilizers has also been cited in the literature, for example, in P. Reichert, H. Nitz, S. Klinke, R. Brandsch, R. Thimann and R. Mülhaupt, Macromol. Mater. Eng., 275, 8-17 (2000).
U.S. Pat. No. 6,271,297 (Ishida) describes the use of a swelling agent, in particular epoxy monomers, caprolactam, or a combination thereof to promote the intercalation and/or exfoliation of a sheet silicate and/or sheet silicone. However, little consideration has been given to the compatibility between such guest molecules/swelling agents and the polymer matrix, which plays a very important role in the reinforcing effect of the nano-reinforcement.
U.S. Pat. No. 6,239,195 (Suzuki et al.) describes the use of silanes as compatibilizers in nanocomposites, wherein a phyllosilicate is pre-treated with a silane compound following intercalation of the phyllosilicate, which treatment further expands the interlayer spacing, thereby facilitating exfoliation. However, this process is more expensive and less practical.
Modified polymers, such as maleic-anhydride-grafted polymers (MA-g-polymers), or copolymers, such as styrene-maleic anhydride copolymers (SMA) are popular compatibilizers or coupling agents for conventional polymeric composites and blends. The use of such polar functional oligomers as compatibilizers in nano-silicate systems has been described. For instance, the use of a hydroxy-functionalized polypropylene oligomer and an organoclay in the preparation of a polypropylene/clay nanocomposite has been disclosed by A. Usuki, M. Kato, T. Kurauchi, J. Appl. Polym. Sci., 63, 137 (1997). The use of polypropylene, a maleic anhydride-modified polypropylene oligomer and stearyl-ammonium-intercalated clay in the preparation of a polypropylene/clay nanocomposite was disclosed by M. Kawasumi, N. Hasegawa, M. Kato, A. Usuki and A. Okada, Macromolecules, 30, 6333 (1997). However, such coupling agents are not well suited as compatibilizers for use in nanocomposite systems, which have a different interaction mechanism and chemistry compared to conventional systems. At low concentrations of the polar groups, these compatibilizers have been known to be ineffective, while at high concentrations, they may form a separate phase thus contributing to undesirable properties which affect the overall performance of the nanocomposites. As a result, such nanocomposites may be less tough and ductile, have poor thermal stability, and lack the desired color.
To optimize the interaction between the non-polar hydrophobic polymer matrix and the hydrophilic nanoclay platelets it is desirable to design a new type of compatibilizer having a suitable structure and chemistry for the formation of nanocomposite systems.
In the context of the above discussion, compatibilizer means an agent capable of interacting with hydrophilic nano-reinforcing materials and at the same time being miscible or thermodynamically compatible with hydrophobic polymer matrices. In this the present application, the word “compatible” is used to indicate either a thermodynamic miscibility of the organic components or positive interactions between the organic and inorganic components, which results in a non-positive value of the free energy of mixing.